Secret signaling system



c. E. ATKINS SECRET SIGNALING SYSTEMv Filed Oct. 2. 1943 July 23, i946.

, INVENTOR CA EL EDWARD ,4me/MS j@ A 1'" y l QM' Maj# JMW ATTORNEYS Patented July 23, 1946 SECRET SIGNALING SYSTEM Carl Edward Atkins, Evanston, Ill., assignor to Tung-Sol Lamp Works, Inc., Newark, N. J., a

corporation of Delaware Application October .2, 1943, Serial No. 504,679

9 Claims. l

My present invention relates to secret stationtostation signaling systems of the general type disclosed .and Claimed in my Ycopending application Serial No. 443,898, led May 21, 1942, and comprises an improved and simplied system of that type particularly adapted for secret telesraphc communication.

'In my present system, as in that of my above mentioned application, secrecy is obtained by scrambling the signal introduced at one station with the signal introduced at the other station and detection of the communicated signal is obtained by cancellation from the incoming energy of the modulation corresponding to the signal previously introduced at the same station,

For an understanding of the improved system of the present invention reference may be had to the specific and preferred embodiment of the invention employing frequency modulation illustrated Yin the drawing, of Which- Fig. l is a diagrammatic circuit drawing of the apparatus at either of two communicating stations equipped with a secret signaling system embodying the invention;

Fig. 2 is a diagram of the special network of Fig. l at one station; and

Fig. 3 is a diagram of the special network of lig, 1 at the other station.

shown in Fig. 1, the equipment at each station in the specic embodiment of the invention illustrated includes a receiving antenna I, a wave trap 2 tuned to the frequency of the local transmltter, a receiver 3 tuned to the frequency transmitted at the cooperating station, a limiter 4, an amplifier 5 and a frequency detector 6 the output circuit of which includes a phase shifting network comprising a capacity 1 and adjustable resistance 8 connected in series and .Capacity 9 connected in shunt across a resistance I0. A resistance II, grounded at one end, is connected at its other end to a tap on resistance II) and to the control grid of a triode I2, The cathode of tube I2 is suitably biased by a network I3 as is also the cathode of a triode I4 connected in tandem with tube I2. The pair of tubes I2 and I4 operate, as hereinafter more fully described, to cancel the frequency dQVations corresponding to the signal previously introduced at the local station and hence may be termed a cancellation mixer. The plates of tubes I2 and I4 are parallel connected and the output from this pair of tubes, which appears across a tuned circuit I5, connected to a source of power through a lter I6, is fed through a blocking ,condenser I'l to a pair of parallel connected resistors I8 and I9. A tap on resistor I8 connects to any suitable audio frequency amplier and reproducer (not shown) such as a loud speaker or the like, while a tap on resistor I9 is connected to the control grid of a triode 20 connected in tandem with a ytriode 2|. The cathodes of tubes 20 and 2| are biased by the network I3 and the plates of this pair of tubes are connected in parallel to a tuned circuit 22 which is connected with the power source through the filter I5. As hereinafter described, tube 2| operates to add the local signal to that introduced at the cooperating station and hence this pair o f tubes may be termed the injection mixer. The output of the pair of tubes 20 and 2|, appearing across the tank circuit 22 is fed through a blocking condenser 23 to a reactance tube 24 which controls the frequency of an oscillator 25. The output from oscillator 25 energizes a power ampliner 26 and transmitting antenna 2l.

For the introduction of the signal each station includes an oscillator 28 operating at a low frequency of say cycles per second, a key 29 and a special network 30 which includes phase shifting means and which delivers a portion of the signal energy through a delay network 3| to the control grid of tube I4 and another portion of the signal energy to the control grid of tube 2 I.

Network 3| may be any known type of delay network, as for example a resistor-condenser combination or an inductor-condenser combination, and network 30 may be any network which includes suitable phase shifting means and preferably also at least one o f the stations amplitude control means. Before describing specic circuits suitable yfor use as the network 30, which circuits are shown in Figs. 2 and 3, the operation of the system as a whole will be described, For convenience in understanding the operation, the yunits at one station, say station A, will be identified by the subscript a and the units at the other station, say station B, will be identied by the subscript b. At station A when key 29a is operated, a part of the energy is supplied through network 30a to the grid of tube 2 Ia, whereupon the output from tube 2| a appearing across tank 22a modulates the reactance tube 24a and a frequency modulated signal is transmitted from antenna 21a. At station B the transmitted energy is received and amplied and then demodulated in detector 5b The phase of the signal energy is shifted by the network lb-b-Sb and a suitable portion thereof applied from resistor IIlb to the grid of tube |212. An amplified signal appears across tank |5b and is applied through blocking condenser I'Ib and control I9b to the grid of tube 2Gb. Appearing yin livered through network 30a to delay network 3|ct' has been delayed in the latter network and is applied to the control grid of tube Marsimultaneously with the arrival of the returning signal to the grid of tube |2a andin phase opposition f thereto. Consequently there will be no corresponding output across tank |5a, If, however,

Cil

key 29h has been depressed during this interval, Y the phase angle of the incoming signal will rbe,Y 1

such as to yield an output across |5a which can be heard as an audio signal at station A and which is also applied to the grid of tube a for addition across tank 22a to the continuing signal delivered to tube 2|a from network 30a. The operation at station B, including the cancellation of the locally introduced signal by tubes |2b and |4b and the addition of the signal by tubes 2|Jb and 2| b is of course the same as above described for station A.

InV the system of Fig. 1 the network 30a and h may be identical but I prefer, however, in order to provide a iiexible system with a wide range Yof adjustment of the phase shift and of the amplification, to employ for one network 30 the circuit shown in Fig. 2, and for the other network 30 the circuit shown in Fig. 3. As shown in Fig. 2, one network 3|] includes a master resistor 32 grounded at one end and connected at its other end to the key 29 and a pair of subsidiary resistors 33 and 34 connected in parallel to van adjustable tap on resistor 32. A tap on resistor 33 supplies energy to the primary of a transformer 35 the secondary of which energizes the grid of tube I4 through a polarity reversing switch 36 and delay network 3|. A tap on resistor 34 connects through a phasing network 31 to the grid of tube! I. e Y

Unit 30 of the other station, as shown in Fig. 3, includes a masterresistor 38, a'tap on which isV connected through a phase shifting network including a resistor 39, condenser 40 and resistor 4|, to the grid of a triode 42. Triode 42, which is suitably biased by a network 43, amplies the signal energy and changes its phase by 18o". The output circuit of triode 42'includes the primary of a transformer 44 the secondary of which is connected to parallel connected resistors 45 and 46. A tap on resistor 45 is connected to th'e grid of tube |4 through delay filter 3| and a tap on resistor 46 is connected to the control Vgrid of tube 2|. Y

For eiiicient operationof the above described system the oscillators 28a and 28h should be synchronized and their outputs maintained in phase. Known stabilizing means could be provided at each station for this purpose o1' the frequency of one oscillator could be locked to that of the other by the transmission from one station to the other of a modulatingV tone asV disclosed in a copending application of mine, Serial No. 501,529, iiled September 8, 1943.v The networks '|-8-,9, 39-40-4| ,'31 and 3|, together with the Apolarity reversing switch 36, provide ample means for adjusting the phase of vthe signal energy aty any point inthe system, the amplifier 5 and triode 42 provide amplification,

amount in the signal energies to be mixed when the amplitudes Vare equal enhance the secrecy of th'e system vasit insures that there will be no change (in amplitude in the radiated signal irrespective of whether one or both keys 29 are depressed.

From the foregoing description it will be apparent that the present invention provides an eiiicient secretY station-to-station signaling sys-v tem wherein the signal originating at one station is scrambledk with. the signal introduced at the oth'er station whereby simultaneous introduction of keyed intelligence at the two stations produces a conglomerate of characters on a more or less capricious basis, resulting in a signal unintelligible to an eavesdropper. While the preferred form of the invention utilizes frequency modulation as a'means for carrying the signal energy to and from the communicating stations, amplitude modulation or phase' modulation can likewise be employed. Y' v The following is claimed:

1. A secret station-to-station signaling system comprising in combination equipment at eachstation Vincluding receiving means, detectingv means connected toreceive energy from said receiving means, a pair of electron tubes connected inV tandem, a phase-shift network between said detecting means and the control grid of one of said tubes, signal introducing means, means for creating a delayed replical with' shifted phase of, theY signal introduced by said introducing means and for applying the same to the control grid ofthe other one of said pair of tubes, a second pair of tubes connected in tandem, a tuned output circuit `for said first pair of tubes connected to the 'control grid of one 0f said second pair of tubes, means for impressing signal energy from said said signal introducing means to the control 'grid of the other one of said second pair of tubes, transmitting means and means for modulating the energy transmitted by said transmitting means by the output from saidl second pair of tubeswhereby said rst pair of tubes may 'be operated as aV cancellation mixer'tocancel from the incoming energy` atene-stationenergycorrresponding tothe signal previously introduced' into 'the system at the same station and said second pair of tubes may be used to scramble the' outgoingv signal withrthe signal introduced at the other station.Y Y

2.."In a secret stationto-station signaling system of the type wherein signal energygintroduced at onestat'ion and transmitted Vtherefrom to the other station isfrriiired at the -latterstation with signal energy introducedatthe latter station, the improvement which comprises atv eachstation a pair of electrontubes Vconnected in tandem, means` for impressing upon thegrid of one of said ,tubesY demodulated energy received from the other station,;means f orirn'pressing upon theY grid ofthe.v other of `,said tubes a delayed replica ofthe signalenergy introduced at the same station for canceling in the output oi said tubes that portion of the demodulated energy received from the other station but previously originating at the same station, a second pair of electron tubes connected in tandem, means for energizing the grid of one of said second pair of tubes from the output of said rst pair of tubes, means for energizing the grid of the other of said second pair of tubes With signal energy introduced at the same station, and means connected to the output circuit of said last pair of tubes for modulating the energy transmitted from the station.

3. The method of operating a secret station-tostation communication system which comprises at each station dividing the signal energy into two portions, delaying one portion and shifting the phase thereof, scrambling the other portion with signal energy introduced at the other station, utilizing the scrambled portion to frequencymodulate the energy transmitted from the station, and utilizing the delayed phase shifted portion for separating the signal energy introduced at the other station from the energy received and detected at the same station.

4. The method of operating a secret stationto-station communication system which comprises at each station dividing the signal energy into two portions, delaying one portion and shifting the phase thereof, scrambling the other portion with signal energy introduced at the other station, utilizing the scrambled portion to modulate the energy transmitted from the station, utilizing the delayed phase shifted portion for separating the signal energy introduced at the other station from the energy received and detected at the same station, maintaining the signal energies introduced at the two stations equal in amplitude and adjusting the relative phase of the signal energies to be 120 out of phase when scrambled.

5. In a secret station-to-station signaling system of the type described, the improvement which comprises at each station a cancellation mixer and an injection mixer, said cancellation mixer including a pair of electron tubes connected in tandem, means for applying to the control grid of one of said tubes energy having components corresponding to the signal previously introduced at the same station and the signal introduced at the other station, means for applying to the control grid of the other tube energy corresponding to the signal previously introduced at the same station but 180 out of phase with the corresponding component of energy applied to the control grid of the rst mentioned tube and said injection mixer comprising a second pair of electron tubes connected in tandem, means for energizing the control grid of one of said last mentioned pair of tubes with the output of the cancellation mixer and means for energizing the control grid of the other one of said last mentioned pair of tubes with signal energy introduced at the same station.

6. In a secret signaling system of the type described, the combination at each station of a low frequency source of signal energy, a key, a network receiving energy from said source under control of said key, said network including phase shifting means and means for dividing the energy received from the source into two portions, means for scrambling one of said portions with energy received from the other station and corresponding to av portion of the energy of the source at the other station, transmitting means, means for modulating the energy transmitted by said transmitting means with said scrambled Portions, means for delaying said other portion, receiving and detecting means, and means for utilizing said delayed portion for unscrambling the detected energy received from the other station.

7. The combination according to claim 6 wherein said network at one station includes a resistor, grounded at one end and connected to said key at its other end, a pair of resistors connected in parallel to a tap on said rst mentioned resistor and a pair of phase shifting circuits each connected to a tap on one of said last mentioned resistors, whereby one portion of the signal energy is delivered through one of said phase shifting circuits and the other portion through the other of said phase shifting circuits.

8. The combination according to claim 6 wherein the network at one of said stations comprises a resistor grounded at one end and connected to said key at its other end, a phase shifting circuit connected to a tap on said resistor, a triode energized from said phase shifting circuit, and a pair of parallel connected resistors in the output of said triode whereby said portions of signal energy may be tapped from said last mentioned resistors.

9. The combination according to claim 6 Wherein the phase shifting means of said network at one station are arranged to shift the phase of one of the portions of energy relative to that of the other portion of energy and the phase shifting means of the network of the other station are arranged to shift the phase of the signal energy prior to division into portions.

CARL EDWARD ATKINS. 

